Amending the U.S. Constitution occurs infrequently only twenty-six times so far in well over two hundred years and is often the result of extraordinary circumstances or shocks to the socio-political system.
Article V of the Constitution specifies the two-stage process by which amendments are adopted. Essentially it requires (1) agreement by two-thirds of the members of each house of Congress in order to formally propose an amendment and (2) adoption of the proposed amendment by three-fourths of the states in order for it to be officially ratified (accepted) as an addition to the Constitution.
A critical period of extraordinarily prolific amendment activity occurred during a seven-year period from 1864 to 1870. During this period the 13th (abolition of slavery), 14th (due process and other clauses) and 15th amendments (suffrage for African Americans) were proposed and passed by Congress, and then ratified by the states.